The present invention relates to an optical amplifier based on erbium-doped fibers and having a gain flattening filter and also to a fiber optical network including such an optical amplifier.
Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. Of course, there is a desire in the existing telecommunication networks to use the available optical fibers in their networks as efficiently as possible, in particular for communication over long distances, since such fibers obviously have high installation costs. By introducing wavelength division multiplexing WDM in existing communication systems using optical fibers and in new communication systems to be built a plurality of individual wavelength channels can be transmitted on the same optical fiber and thus the information transmitted over the fiber can be multiplied.
In optical fiber networks for long distance communication there may be a need for amplifying the optical signals. Such amplification can of course be achieved by a repeater built in a straight-forward way, including components converting the optical signals to electrical signals, amplifying the electrical signals and converting the electrical signals to optical signals. For WDM signals this will require one optoelectrical and one electrooptical converter per wavelength channel used in the WDM transmission and also one filter or demultiplexer for filtering out the different wavelengths in the incoming signal. This will obviously be very costly and also results in reliability problems owing the large number of components, both electronic and optical, which are required.
Another type of amplifier comprises optical fiber amplifiers based on optical fibers doped with rare-earth metals, primarily erbium-doped fiber amplifiers. Such amplifiers have great advantages when used in optical fiber systems owing to e.g. their compatibility with the optical fibers and their high gain, and they are in particular advantageous when used in wavelength multiplexed transmission systems, since they are capable of simultaneously amplifying a number of WDM channels and only require a limited amount of electronic components. The basic design of an erbium-doped fiber amplifier includes one length of an active, erbium-doped optical fiber, connected at its input end to the output of a 2-to-1 optical coupler, the coupler receiving on one of its inputs the signal to be amplified and on the other input more energetic light providing the power for amplifying the signal. This more energetic input light is called the pump light and is obtained from an optical power source, called the optical pump. The pump light has a shorter wavelength tan that of the signal and is generally more energetic and is capable of lifting erbium ions from lower energy states to higher energy states in the erbium-doped fiber. Light is then generated in the fiber when the ions return to lower energy levels.
However, in erbium-doped amplifiers a problem related to spontaneous emission may exist. Spontaneous emission results from the interaction of the pump light with the erbium metal ions in the fiber and appears as noise added to the signal to be amplified. Furthermore, the light caused by spontaneous emission is amplified during the propagation of light in the fiber, in both directions, resulting in so called amplified spontaneous emission ASE. The ASE is very nearly proportional to the amplifier gain and therefore the ASE spectrum is very similar to the gain spectrum, having a preponderance of power at the gain peak.
In U.S. Pat. No. 5,375,010 an optical amplifier is disclosed comprising two lengths of erbium-doped fiber connected in series via an isolator. The isolator reduces the transmission of backward-travelling amplified spontaneous emission ASE from the second length to the first length when optical pump power is supplied to the first length.
U.S. Pat. No. 5,260,823 discloses an erbium-doped fiber amplifier having a gain-shaping filter between two lengths of erbium-doped fiber. The filter is a band-rejection filter operating at the peak wavelength of the gain spectrum. The attenuation of the band-rejection filter is chosen so that it exactly cancels the larger gain at the peak wavelength and thus modifies the overall gain spectrum to a more uniform shape. Typical filter values comprise an 8 dB attenuation at 1531 nm, for a 3 dB-bandwidth of 4 nm. A filter used is based on wavelength-selective resonant coupling from the propagating core mode of the fiber to a cladding leaky mode. A single optical filter used was obtained by periodically perturbing the fiber and it consisted of a grating having a period of 775 xcexcm, the fiber being sandwiched between the grating and a flat plate. Also a dielectric interference filter can be used.
Long-period grating filters have been proposed for gain-flattening erbium-doped amplifiers. In U.S. Pat. No. 5,430,817 long period grating filters are placed at each end of a length of amplifying fiber pumped in opposite directions at two different wavelengths, the filters removing unused pump energy which has passed through the length of amplifying fiber. In the article Paul F. Wysocki et al., xe2x80x9cBroad-band Erbium-Doped Fiber Amplifier Flattened Beyond 40 nm Using Long-Priod Grating Filterxe2x80x9d, IEEE Photonics Techn. Lett., Vol. 9, 10, October 1997, a long period grating filter is applied between two lengths of erbium-doped fiber, the lengths being individually pumped at different wavelengths. In R. Lebref et al., xe2x80x9cTheoretical Study of the Gain Equalization of a Stabilized Gain EDFA for WDM applicationsxe2x80x9d, J. Lightw. Techn., Vol. 15, No. 5, May 1997, gain-flattening is theoretically studied for a case having a first band-rejection filter inserted between two lengths of erbium-doped fiber and a second band-rejection filter connected to the output end of the doped fiber, the first filter being more dissipative than the second one. The filters can be long period fiber gratings having approximately Gaussian characteristics. The first filter had a maximum attenuation of 5.2 dB at 1531 nm for a bandwidth of 7 nm and the second filter had a maximum attenuation of 3 dB at 1533.4 nm for a bandwidth of 5 nm. The first filter should be inserted after the first quarter of the total doped optical fiber in order to get the lowest noise figure or after the first tenth in order to get best stabilized gain, the latter case however resulting in a bad noise figure. Fabrication of long period fiber gratings is e.g. described in A. Vengsarkar, xe2x80x9cLong-period fiber gratingsxe2x80x9d, in Conf. Optical Fiber Communications, 1996 Tech. Dig. Ser. Washington DC: Opt. Soc. Amer. 1996, Vol. 2, pp. 269-270, paper ThP4.
It is an object of the invention to provide an optical fiber amplifier having a low noise figure and a high gain.
The problem solved by the invention is thus how to design an optical fiber amplifier having its gain flattened in the conventional way, maintaining a high gain and a low noise figure.
An optical fiber amplifier using as an active medium a length of optical fiber doped is with some rare-earth metal such as erbium is designed to amplify light signals within some useful wavelength band, for instance in a range of 1540-1555 nm. The active optical fiber receives in the conventional way light signals to be amplfied and pump light of a first pump wavelength from a pump source. A gain flattening filter is connected in the active optical fiber length in order to give the amplifier a gain which is substantially constant for all wavelengths within the useful wavelength band. A noise filter is also connected in the active optical fiber in a position, which is not located more closer to the output end of the active fiber than the gain flattening filter. The active fiber length can then be divided in three portions which are serially connected and in the joints of which the filters are connected. The noise filter effectively blocks light waves having wavelengths around the gain peak of the spontaneous emission and the gain filter attenuates wavelengths within the useful signal band in order to produce a flat gain curve. The introduction of the noise filter increases the total amplification of the amplifier, maintaining a low noise figure.
Additional objects and advantages of the invention will be set forth in the description which follows, and in pan will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.